1. Field of the Invention
This invention relates to a treatment method of cleaning plastic molded items for high-purity processes required, for example, in the semiconductor industry and chemical analysis such as fluororesin molded items.
2. Description of the Prior Art
Since fluororesins are very excellent in chemical resistance and do not require additives such as plasticizers and stabilizers, and metal ions do not mix in fluororesins during the production of the fluororesins, organic materials or metal ions bleed far less from fluororesins. Therefore, fluororesins are widely used as material for apparatuses such as containers that will be in contact with high-purity water or high-purity liquid chemicals and in particular PTFE (polytetrafluoroethylene) and PFA (tetrafluoroethylene/perfluoroalkyl vinyl ether copolymers) are used favorably as material capable of withstanding liquid chemicals at high temperatures.
As metal impurities contained in resin powder or pellets of these fluororesins that are used as raw materials for molding, Na, Fe, Cr, Ca, and Al are, for example, detected and in particular Fe and Na are noticeable and amount to several tens of ppb. When these fluororesin molded items are immersed in a high-purity treatment liquid for several tens of minutes, the amount of metals extracted from the surface layer sections of the molded items, the metals being originated from the raw materials of the molded items themselves, is far less, and even if all metals present to a depth of 10 .mu.m from the surfaces are extracted, the amount is on the order of 0.01 ng/cm.sup.2.
However, when molded fresh items of PTFE or PFA are boiled in pure water for several tens of minutes frequently, Na, Fe, Cu, etc. are extracted in an amount of 0.1 ng/cm.sup.2 or more, and even if the boiling is continued, the extraction phenomenon does not stop easily. Presumably when an item is molded, metal impurities contaminate the surfaces of the molded item and are absorbed into a very shallow layer beneath the surfaces.
Consequently, as a method of eliminating effectively metals which will be extracted from the inside of a fluororesin molded item to be used, for example, in an ultramicro analysis, in the case where the molded item is new, the following steps:
(1) cleaning with soap or a synthetic detergent, PA1 (2) cleaning with acetone, PA1 (3) heating in hot concentrated nitric acid for 3 to 5 days, and PA1 (4) heating in hot 0.1N nitric acid for 5 days are successively carried out with each of the above steps followed by rinsing in pure water, which means that a very long-period treatment is required (see "Bunseki" published by The Japan Society for Analytical Chemistry, October, 1984). This cleaning method uses the function that nitric acid which can well dissolve metal impurities are absorbed from the fluororesin surface to the inside.
Although the degree of absorption of chemicals .into fluororesins differs from agent to agent, other acids often used in dissolving samples in analysis, for example, hydrochloric acid and aqua regia are also well absorbed similarly. Also hydrofluoric acid and SC-2 cleaning solution (hydrochloric acid: hydrogen peroxide: water=1:1:5 in volume) of RCA most commonly used in cleaning in wafer processing in semiconductor factories are well absorbed similarly.
PFA is often used as a wafer carrier in a semiconductor washing step since PFA is a chemical resistant and heat resistant fluororesin which can be melt-molded. However, as described above, the treatment with the acids results in absorption of the acid and in order to eliminate the absorbed chemicals, conventionally, running pure water rinsing is carried out for a long period of time.
However, in such a pure water rinsing treatment, it is difficult to eliminate the absorbed chemicals satisfactorily and as the carrier is used repeatedly, the remained substances accumulate and evaporate to cause detrimental defects on the wafer surface in some cases, which is a phenomenon called "chemical carry-over".
When such wafers are washed, metal impurities contaminating the wafers transfer to the washing liquid and these metals are absorbed along with the absorption of the washing chemical into the fluororesin and accumulate as the fluororesin carrier is used repeatedly. The metal impurities thus accumulated cannot be eliminated by washing with water. When the thus contaminated fluororesin carrier is put in another high-purity washing liquid, the adsorbed metal impurities are extracted out to hinder the washing effect to wafers considerably.
Where apparatuses of PTFE and PFA are used in an ultramicro analysis, if a sample to be analyzed is a metal, a high-pure acid used for dissolving it is contaminated with the major component element. As a consequence, if these apparatuses are used again, it is required that they are washed thoroughly to remove the absorbed metals. According to "Bunseki" mentioned above, an apparatus in use is required to be treated by washing with hot nitric acid for 3 days and with hot 0.1N nitric acid for about 3 days. Such a treatment is possible for research but is quite inefficient in practice.
Generally, the phenomenon that impurity ions are absorbed from the surfaces of a molded item into the inside can be handled as an inward diffusion phenomenon where the surface concentration is constant so long as the ions do not change chemically. The distribution of ions can be indicated by the complementary error function. Since the diffusion coefficient of ions in a fluororesin is considerably small, where the time of the immersion of the fluororesin item in a treatment liquid is short, the region where the concentration of the absorbed ions is high is limited to a shallow layer near the surface. Therefore, where a fluororesin molded item is used repeatedly in chemicals, it is recommended that absorbed ion eliminating treatments which use outward diffusion are effected frequently. Heating the ion-absorbed resin item in pure water is considered in the case of outward diffusion of the treatment liquid; in a diluted aqueous solution of chemicals capable of dissolving highly the ions such as nitric acid in the case of outward diffusion of metal impurity ions is naturally considered. If impurity ions are adsorbed chemically to the fluororesin or are reduced to metal atoms, it is required that a powerful chemical such as concentrated nitric acid capable of ionization to allow them to go to an aqueous diluted agent solution for extraction is caused to be absorbed into the resin previously.
The cleaning method of the above prior art is based on this technical background. In practice, since there is contamination with fats and oils during the production and transportation, degreasing is effected previously with a detergent or an organic solvent in order to make the chemical treatment as mentioned above effective.
In other words, the conventional cleaning method based on the concept as described above takes a longer treating time and therefore is not suitable practically for frequent cleaning of fluororesin molded items which are used at all times.